@programminginc - страница 2482

Elnee

sad

Necrobumper

klimi

Necrobumper

Indeed

Ибраги́м

Lol I just measure how many lines there and make shallow judgments xD

Don't worry dear, u've made worse life decisions.

MᏫᎻᎯᎷᎷᎬᎠ

And type? Possibly... Not sure

yeah it has the type information such as size to calculate the access so if your type size is 4 and you want to access the 3rd element the calc is just 3 * 4 = the index of the array

MᏫᎻᎯᎷᎷᎬᎠ

Don't worry dear, u've made worse life decisions.

Really? My life was going very well this far

Anonymous

template <typename _Up, typename _Ep, typename = void> struct _Ptr { using type = _Up*; }; template <typename _Up, typename _Ep> struct _Ptr<_Up, _Ep, __void_t<typename remove_reference<_Ep>::type::pointer>> { using type = typename remove_reference<_Ep>::type::pointer; }; this is in __unique_ptr_impl of libstdc++. i don't understand line 6 Edit: nvm i got it

Nils

How do I execvp() without replacing the current process?

Alex

How do I execvp() without replacing the current process?

use fork

Nils

use fork

but how do I tell the fork what to run?

Nils

Can I pass the some pointer to fork()?

Alex

or try system

Nils

or try system

Nope, it's too unsafe.

Alex

but how do I tell the fork what to run?

inside child process call execvp

Nils

Nope, it's too unsafe.

(I am involving user input...)

Nils

inside child process call execvp

but how does the child know that it's a child and which arguments to pass to execvp?

vinícius*

you can tell childs by the PID value

vinícius*

the child will have PID == 0

Alex

but how does the child know that it's a child and which arguments to pass to execvp?

fork returns zero for child, and pid of child for parent

Nils

fork returns zero for child, and pid of child for parent

but how do I pass the arguments for execvp() to the child?

Nils

Do I need a fd or something like that?

vinícius*

the child will have access to all the variables the parent process had at the moment of forking

Nils

the child will have access to all the variables the parent process had at the moment of forking

so they won't be reinitialised?

vinícius*

nah

Alex

but how do I pass the arguments for execvp() to the child?

you can read from stdin in child or use IPC to pass data from parent

vinícius*

you're trying to make a shell ?

Nils

you're trying to make a shell ?

nope.

vinícius*

well, you can do something like this: char ** aux = malloc(read_from_pos * sizeof(char *)); for(i=0; i<read_from_pos; i++) aux[i] = parsed[i]; if (fork() == 0) { execvp(aux[0], aux); fprintf(stderr, "%s: command not found.\n", aux[0]); }

Nils

well, you can do something like this: char ** aux = malloc(read_from_pos * sizeof(char *)); for(i=0; i<read_from_pos; i++) aux[i] = parsed[i]; if (fork() == 0) { execvp(aux[0], aux); fprintf(stderr, "%s: command not found.\n", aux[0]); }

that has to be inside my main(), right?

vinícius*

nope, anywhere

Nils

nope, anywhere

so fork() always calls the function that it is called from?

vinícius*

fork will always run from where it's called

Nils

fork will always run from where it's called

nice, thx

Nils

fork will always run from where it's called

Will it have the same fds too?

vinícius*

fork will always run from where it's called

I mean, the exact same line

Nils

I mean, the exact same line

oh.

Nils

Will it have the same fds too?

^

vinícius*

yeah

vinícius*

it'll inherit the file descriptors of the parent process, afaik

vinícius*

you wanna use dup2?

Nils

you wanna use dup2?

I want to somehow read the process output

Nils

But I am too much of a beginner in file descriptors, tbh

vinícius*

I want to somehow read the process output

as in saving it to a file or printing it to stdout?

Nils

as in saving it to a file or printing it to stdout?

Storing it inside a char *

vinícius*

I imagine you can do something like this:

vinícius*

int fds[2]; const char *argv[3] = {"/bin/ls", "-li"}; pipe(fds); if (fork() == 0) { close(fds[0]); dup2(type[0] == 'r' ? 0 : 1, fds[1]); close(fds[1]); execvp(argv[0], argv); exit(-1); } close(fds[1]); FILE * proc_output = fdopen(fds[0], type);

vinícius*

the child's output should be contained within that file, so you can read it with fscanf, fread, whatever

Nils

Can I wait for the fork()ed process to having exited?

vinícius*

yeah

Nils

yeah

just wait(pid)?

vinícius*

wait(0), yup

Nils

thx

Nils

int fds[2]; const char *argv[3] = {"/bin/ls", "-li"}; pipe(fds); if (fork() == 0) { close(fds[0]); dup2(type[0] == 'r' ? 0 : 1, fds[1]); close(fds[1]); execvp(argv[0], argv); exit(-1); } close(fds[1]); FILE * proc_output = fdopen(fds[0], type);

struct databuffer db_fromcmd(const char **command) { struct databuffer outdb; struct databuffer readdb; db_init(&outdb); int stdout_backup = dup(1); dup2(memfd_create("pipe", 0), 1); if (fork() == 0) { execvp(command[0], command); } FILE *proc_out = fdopen(1, "r"); while (!feof(proc_out) == 0) { char outb[16]; fread(outb, sizeof(outb), 1, proc_out); readdb = sized_db(outb, sizeof(outb)); db_append(&outdb, &readdb); } wait(0); close(1); dup2(stdout_backup, 1); return outdb; }

Nils

Right?

Nils

Oh wait I should not forget to dup back the original stdout

Nils

Done

Nils

struct databuffer db_fromcmd(const char **command) { struct databuffer outdb; struct databuffer readdb; db_init(&outdb); int stdout_backup = dup(1); dup2(memfd_create("pipe", 0), 1); if (fork() == 0) { execvp(command[0], command); } FILE *proc_out = fdopen(1, "r"); while (!feof(proc_out) == 0) { char outb[16]; fread(outb, sizeof(outb), 1, proc_out); readdb = sized_db(outb, sizeof(outb)); db_append(&outdb, &readdb); } wait(0); close(1); dup2(stdout_backup, 1); return outdb; }

execvp() seems to never be called, any idea?

piggyho

Hey. after python, I obviously don't understand how this language works. Why can I access a nonexistent index in an array? where did the given value in this array come from?

bounds checking reduces performance in c/cpp, an array is contiguous memory. and the index is just an offset from the start don't confuse that with std::array

Anik

can u explain me how can i imorove C+

Anonymous

Coding

数学の恋人

Coding

Nice

q1248 •卜

great

/home/aleksandr/

bounds checking reduces performance in c/cpp, an array is contiguous memory. and the index is just an offset from the start don't confuse that with std::array

Thanks 👍

Nils

Hey. after python, I obviously don't understand how this language works. Why can I access a nonexistent index in an array? where did the given value in this array come from?

That's simple, it's called undefined behavior. Avoid accessing out of bounds data as it might make your program go crazy or crash.

Nils

If you want truly efficient bound checking use c++ std::vector

/home/aleksandr/

Okey, thanks :)

Nils

Okey, thanks :)

Or, if you need the bound check while iterating only, use a so-called "null-terminator"

Nils

It is a special byte that marks the end of the array

/home/aleksandr/

I will consider, thank

Liam

Hey. after python, I obviously don't understand how this language works. Why can I access a nonexistent index in an array? where did the given value in this array come from?

In C/C++, raw array stores its data sequentially in memory. When accessing elements in array by arr[i], C/C++ will try to dereference data stored in arr + i, with given type. The behavior is undefined, according to the standard, which means it may cause segmentation fault, or may return as usual, or may do anything else. Programmers are responsible for boundary checking, in these cases, in C/C++ world. In your case, xl + 88 is a valid mem-address for your program, hence no seg-fault caused by dereferencing it.

Anonymous

Hello, how can we use c++ for hacking? Is there any good website or something. I know c++ already

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What